The invention relates to a boron aluminosilicate glass for optical applications which in particular can be used advantageously as a core glass in optical fibers.
Prior art optical glasses having a refractive index between 1.54 and 1.62 (extra dense crown region and light barium flint region) usually contain PbO to reach the desired optical characteristics (refractive index of 1.54≦nd≦1.62 and an Abbe number of 48≦vd≦57) and a good transmission. Such glasses are of interest for numerous optical applications, e.g. for applications in imaging, projection, telecommunication, optical communication engineering and laser technology, however, in particular also for fiber applications (imaging fibers and/or light transmission fibers). Due to their lead content such glasses offer a low chemical resistance. Also often As2O3 is used as a refining agent. Since within the last years the glass component PbO and often also As2O3 have been regarded as environmentally problematic, the most manufacturers of optical instruments and products tend to use glasses free of lead and arsenic. For the application in products having a high degree of coating also materials of enhanced chemical stability (for undergoing the coating processes without damage) by keeping a high transmission (very low loss or attenuation) are gaining more and more importance.
For replacing lead in classical optical glasses, glasses containing large amounts of TiO2 in a silicate matrix are known that lead to glasses being very instable with respect to crystallization, being difficultly meltable and hardly processable. In addition, the transmission of the glasses deteriorates (the loss increases) due to the intrinsic adsorption of TiO2.
Also lately the development of “short” glasses has been desired due to processing aspects, i.e. glasses the viscosity of which is extremely temperature dependent. This behavior during processing offers the advantage that hot forming times, i.e. the times of mold closure, can be reduced. Thereby throughput can be enhanced on the one hand, and on the other hand it is easy on the mold material, this having a very positive effect on the total manufacturing cost. Also due to the fast cooling (shorter mold closure times) even glasses exhibiting an enhanced crystallization tendency can be processed when compared with longer glasses, and an initial nucleation which would be problematic during subsequent secondary heat-forming steps (fiber drawing, ion-exchange, subsequent pressing, reforming, fine cooling, etc.) is avoided.
For micro-structuring purposes (gradient-index lenses, (flat, rod shaped fiber (bundle)-like) light guides, spherical lenses etc.) using ion-exchange (e.g. (Na—Ag)) the novel materials should also be ion-exchangeable on an economical basis in standard processes. An additional characteristic may be the tension building ion-exchangeability (e.g. Na—K, “pretensioning”).
Optical materials for general applications should also be applicable in fiber applications, e.g. as fiber core glasses. To this end, novel types of glasses must particularly be tolerant against secondary heat-forming steps (fiber drawing, melting together, etc.) and must have a good compatibility with conventional fiber cladding glasses.
Commonly, a glass fiber for the transmission of light comprises a highly refractive core glass and a cladding glass enclosing the core glass and having a lower refractive index than the core glass. Under suitable conditions, stepped-index fibers comprising a core glass and a cladding glass completely enclosing the core glass at its outer peripheral wall can be produced. A light transmissive glass body of fiber shape within which the core glass offers a constant refractive index across the total cross surface is referred to as a stepped-index fiber. Glass fibers of this type transmit light, being introduced at one end of the fiber into the core, to the other end of the fiber, wherein the light is completely reflected at the interface between the core glass and the cladding glass (total reflection).
The amount of light that can be coupled into and transmitted within such a fiber is proportional to the square of the numerical aperture (NA) of the fiber and to the cross-sectional area of the fiber core. For transmitting large light amounts via long or middle distances (≦100 meters), such stepped-index fibers are often packed together to dense fiber bundles, are equipped with a protecting hose, are bonded with their ends to metal shells, and the front surfaces are processed to yield optically flat surfaces by grinding and polishing. Suitably fabricated optical fiber bundles are referred to as fiber optical light guides. In case a production process is used which allows for a geometric arrangement of individual fibers, in this way image light guides can be produced.
The higher NA of the individual fibers within the bundle, the larger amounts of light can be transmitted by these light guides.
Fiber optical light guides are used in various technical and medical applications (general industrial processes, illumination, traffic, automobile, architecture, endoscopy, dental medicine). Their most important function is the transmission of a light stream as large as possible from a place A to another place B, normally via short or middle distances (a few to 100 meters maximum). Herein often light emerging from a high power light source, such as a halogen or discharge lamp, is coupled into the fiber bundle by means of optical aids, such as a lens or a reflector.
The light amount transmitted by fiber optical light guides depends, apart from NA of its fibers, also from the transmissive characteristics of the core glasses contained therein. Only core glasses of very specific compositions having very low contaminations in the raw materials, from which they are molten, transmit the light with low attenuation along the total length of the light transmitter. The raw materials for melting such core glasses are relatively expensive due to the high purity required which may lead to considerable manufacturing costs for such fibers or for such light guides made thereof.
Apart from the amount of light transmitted by a fiber optical light guide, also a color true transmission of the light is of importance in many cases. Due to the spectral transmissive dependence of the core glass which is contained in the fibers, there may be a color deviation in the color position of the feeding light source, which may have a higher or lower degree, this often leading to a yellow color cast of the light emerging from the light guide. This is always detrimental when a color neutral representation is required (e.g. in the medical endoscopy with photographical image documentation for differentiating between healthy and malignant tissue etc.). The manufacture of optical stepped-index fibers from multi-component glasses is performed either in the so-called double-mold process or in the rod-tube process. In both cases, the core and cladding glasses are heated up to temperatures which correspond to a viscosity range between 104 and 103 dPas, and are drawn to fibers. To allow a manufacture of a stable fiber with low loss, the core and cladding glasses must be compatible to each other with respect to a variety of characteristics, such as the course of viscosity, the thermal expansion, the crystallization tendency, etc. In particular, there may be no contact reaction or crystallization, respectively, at the interface between the fiber core and cladding which would considerably impair a total reflection of the light introduced into the fiber core and which would render the fiber unsuitable for an application for low-loss light transmission. In addition, also the mechanical stability of the fiber would be negatively influenced by crystallization.
From U.S. Pat. No. 5,744,409 an optical borosilicate glass for precision pressed parts is known that is a material suitable for high borate amounts (up to a maximum of 30 wt.-%). In addition the glass contains a high amount of lithium oxide (7 to 12 wt.-%.
The high boron content leads to an extension of the viscosity which is detrimental with respect to processing. Also the chemical stability is impaired by a higher component mobility leading to lower crystallization stability and worse ion-exchange characteristics. The high lithium content in combination with a high borate content leads to an increased corrosion attack against the melting end wall material, this leading to increased production cost. If melting is performed in platinum, then the platinum input is increased and thus the transmissivity is deteriorated (not acceptable bad attenuation values). In case melting is performed within refractory material or within silica, then the crystallization stability is deteriorated by the material input (inner pre-nucleation).
From DD 283 281 A3 an irradiation resistant UV-transmissive optical filter glass free of cerium is known that comprises germanate and/or tin oxide and/or antimonic oxide as mandatory components. Here these components serve to stabilize the material against irradiation damages during long-term use. The protective effect thus is based on a masking of the UV-absorption line initiating the damaging. For this purpose, in fact only very small amounts, i.e. doping amounts of the relevant material, are sufficient. However, exactly these characteristics of the afore-mentioned glasses render these not suitable for the applications mentioned at the outline, in particular not suitable as fiber optical material which must have a particularly low attenuation throughout the total spectral region.
From JP 88 008 056 an acid resistant, hydrolytically stable, optical and ophthalmic glass is known that comprises particularly high amounts of network formers in a certain ratio so that a borosilicate glass high on boron having optional amounts of Al2O3 results. The material disclosed herein may in this way contain up to 18 wt.-% B2O3. Such a high borate content must be seen as disadvantageous, since this leads to a strong extension of the viscosity. Also the chemical stability as well as the ion-exchange characteristics are impaired by the higher component mobility. Also negative is the fact that apart from a very high amount of CaO (7 to 30 wt.-%) a mandatory addition of TiO2+Nb2O5 of 10 to 21 wt.-% in total is necessary. Such a high amount of CaO (network modifier) leads to a strong expansion of the stabilizing network and thus to strong crystallization tendencies and low chemical resistance of the glasses. Also, there is a high potential for the generation of diffusion impeding layers in the initial phase of a potential ion-exchange, this impeding an economical and suitable exchange with respect to this application. The addition of TiO2+Nb2O5 in total leads to an extremely strong decrease in transmission at the blue spectral edge, since both components have strong self-absorptions. Also, they decrease the crystallization stability of the material due to their nucleating characteristics.
From U.S. Pat. No. 3,365,315 a process for the manufacture of hollow glass parts having a certain density is known. The glass that is used comprises a very high amount of SiO2 (60 to 80 wt.-%). Alkali oxides and alkaline earth oxides are added only in such small amounts that a suitable short glass cannot be reached.
JP 85 221 338 A (patent abstracts of Japan 60 221 338A) discloses an optical oxy-halogenated glass having a refractive index in the range between 1.62 and 1.85. Herein mandatory amounts of at least 1 wt.-% La2O3 and of at least 0.1 wt.-% Y2O3 are necessary to obtain the optical position. Thereby, the attenuation characteristics of the material are deteriorated by intrinsic absorption and also the glass is made more expensive.
From EP 0 265 983 B1 a luminescent silica glass is known that comprises only a small amount (0 to 0.15 moles of Al2O3, B2O5, P2O5, ZrO2, Sc2O3, Y2O3, La2O3, Gd2O3, ZnO per mole of SiO2). In addition, the glass comprises Tb2O3 and/or Ce2O3 as a luminescence activator.
With such a high SiO2-content and only small additions of fluxing agents or network modifiers, respectively, the desired viscosity profile cannot be reached. Also the added activators are detrimental for the desired application, in particular for fiber applications.
From EP 0 399 577 A1 a high energy irradiation sensitive glass is known that is resistant against actinide irradiation. However, the glass comprises silver ions as well as photo-sensivity inhibitors, this being contradictory to the preferred applications, in particular to fiber applications.
For the manufacture of stepped-index fibers basically three fiber systems are known in the prior art.
The fiber system which is probably the best-known and most common one comprises a core glass of high lead content (commonly 35 wt.-% PbO and an alkali borosilicate glass as a cladding glass. The advantage rests in the high numerical aperture that can be reached (up to more than 0.7 with PbO contents of >50% in the core glass) together with low manufacturing costs and a good capability of drawing to fibers without crystallization problems. This, however, is in contrast to drawbacks such as a mediocre or bad attenuation (200 to 300 dB/km) as well as a somewhat high color cast, mainly caused by Pb self absorption (blue edge of the visible spectral range), as well as dragged-in contaminations of elements highly blue coloring, such as chromium and nickel. Also lead as an environmentally polluting material has come into disrepute more and more and hence is applied for fibers only in specific applications or not at all any more.
A second fiber system comprises an alkali borosilicate glass which is applied as a core as well as a cladding glass.
In the prior art several such glass systems are described, e.g. within EP 0 018 110 B1 or in EP 0 081 928 B1 or in DE 29 40 451 A1 or in U.S. Pat. No. 4,264,131. Partially these glasses, apart from a high boron content, also contain high amounts of alkaline earth and/or zirconium and germanium oxide to reach the desired high refractive index. The advantage rests in the very low attenuation (partially around 10 dB/km) and in their low color cast together with normally environmentally friendly raw materials. A disadvantage of these glasses rests in the commonly lower numerical aperture of the fibers as well as in a lower chemical stability. Also the mandatory boron oxide amount (U.S. Pat. No. 4,264,131, EP 0 081 928 B1, DE 29 40 451 A) is detrimental with respect to the refractory material stability. Due to the lower chemical stability the fibers, during their manufacture, directly after drawing, e.g. from a drawing die at the double mold, must be supplied online with a plastic coating protection against possible chemical and/or mechanical attack. In addition, the low attenuation is achieved only by utilizing highly pure and thereby very expensive raw materials. The two last mentioned aspects, high manufacturing cost and a mandatory plastic coating, thus render practically impossible an application as fiber bundles for broader applications. By contrast, they are used as single fibers for data or energy transfer (laser fiber) in a variety of special applications.
Also fibers on pure Si2O-basis basically are possible as a third fiber system for fiber bundles for the transmission of light. Their advantages resting in an extremely low attenuation (up to 6 dB/km) in a good color neutrality and good environmental compatibility, are in contrast in particular to the high cost. Pure silica glass is extremely expensive due to its high processing temperature. In addition, there is a complicated doping process of the so-called preform according to which by the introduction of fluorine into the surface of a cylindrical rod the necessary reduction in refractory index of the pure quartz is reached that is necessary as an optical isolation to achieve light transfer in the later fiber. Also the numerical aperture of quartz fibers that can be reached is somewhat limited (0.22).